1. Technical Field
This invention relates to a method for separating gas and liquid and cyclone separators therefore.
2. Description of the Related Art
Off-shore extraction of hydrocarbons from a subsea reservoir often involves transportation of a mixture of hydrocarbons, water and dissolved salts in subsea pipelines from the reservoir up to land based or floating top-side facilities for processing the mixture to recover the desired hydrocarbon products. Due to shifting physical conditions during the pipeline transit, there is a problem with formation of hydrates in the fluid mixture of the pipelines threatening to clog the lines.
One much applied solution to the problem of hydrate formation is to add, at subsea level, low water content glycol into the process fluid which usually is a mixture of hydrocarbons, water and dissolved salts and then extract the glycol as so-called rich glycol from the process fluid at the top-side facilities. From an operational costs and environmental point of view, the rich glycol should be regenerated to lean glycol and then reused as hydrate inhibiting agent in the subsea lines. Rich glycol usually contains remains of the hydrocarbons, high water levels, corrosion products (solid particulate corrosion produced remains such as rust flakes etc.) and a mixture of dissolved mineral salts.
From U.S. Pat. No. 6,340,373 it is known a method for the treatment and processing of solutions of an organic fluid, water and one or more compounds of alkaline earth metals, alkali metals and metal ions. The method comprises the following steps: a) conducting a stream consisting of organic fluid, water and one or more compounds of alkaline earth metals, alkali metals and metals ions to a salt reduction and crystallization unit, b) flash evaporating and optionally partially condensing the stream and providing thereby a steam of evaporated water and organic solvent and a steam of organic fluid and the compounds of alkaline earth metals, alkali metals and/or metal ions, c) conducting the steam consisting substantially of organic fluid and the compounds of alkaline earth metals, alkali metals and/or metals ions to a salt reduction means, d) nucleating crystals of the compounds of alkaline earth metals, alkali metals and/or metal ions by means of depressurization and temperature increase, and e) removing the portion of the precipitated crystals and/or particles from the organic fluid. The method is particularly well suited for processing of solution consisting of mono-, di, tri, or polyethyleneglycol or mixtures thereof.
A problem with the processes employing flash evaporation to concentrate and precipitate the salts is that the level of entrained gas bubbles in the liquid in the bottom of the flash evaporator may become unacceptably high, leading to cavitation problems in downstream pumps. The cavity problems may arise when the volume fraction of gas in the liquid extracted from the flash evaporator is from approx. 5 vol % or higher. This problem is especially relevant for liquids of high viscosity, such as is the case for i.e. reclamation of glycol, because entrained gas bubbles are less prone to rise up to the liquid-gas interphase in high viscous liquids and thus escape to the gas phase above the liquid.
Another factor affecting the gas entrainment problem is the gas pressure in the flash evaporator. At lower gas pressures, an entrained gas bubble will become compressed to a higher degree when dragged down into the liquid compared to a similarly sized gas bubble being entrained at a higher gas pressure and dragged down to the same depth in the liquid. For example, in monoethylene glycol, a gas bubble of 1 mm diameter with a gas pressure of 103 Pa just below the liquid-gas interphase will be compressed to a bubble of diameter 0.5 mm at 1 m depth in the liquid. However, if the gas bubble had a pressure of 105 Pa just below the liquid gas interphase, it will only be compressed to 0.9 mm diameter at 1 meter depth. Thus in vacuum flashing, there will be increased problems with small gas bubbles which have significantly smaller buoyancies and thus less capability of being separated from the liquid phase.
One solution of the problem of gas bubble entrainment is disclosed in U.S. Pat. No. 4,375,386 which employs a vacuum evaporator provided with an integral cyclonic-type entrainment separator at the top of the evaporator vapour body which comprises a helical spin plate for imparting a centrifugal action to the vapour and entrained liquid rising from the boiling liquid in the flash chamber. Liquid entrainment is deposited on the vertical wall of the evaporator and flows downward counter-current to vapour flow into a collecting trough, from which it is discharged into a pipe that returns it to the boiling liquid.
U.S. Pat. No. 5,669,948 discloses a cyclone, mainly for separating liquid from gas/vapour which is formed on pressure reduction of spent cooling liquor in connection with pulp production, which includes a casing, an inlet arrangement, a lower outlet line for liquid and an upper outlet line for vapour and gas. The inlet arrangement is connected to a supply line with a valve and the inlet arrangement consists of an exchangeable insertion pipe, the length of which exceeds one meter, preferably two meters and is most preferably between 2.5 and 3.5 meters long. The cyclone is equipped with an “anti-swirl plate” which is arranged on stays directly above the bottom inside the cyclone. The plate is divided up and consists of an external annular part, on top of and in front of which a homogeneous circular part is arranged. Due to the anti-swirl plate, the liquid which accumulates in the bottom of the cyclone cannot be carried away by the swirl which is formed in the central parts of the cyclone.
Another problem associated with cyclone separators is the transfer of the swirling motion into the bulk liquid which may result in a vortex flow into the liquid outlet, thereby resulting in gas leaving the cyclone together with the liquid.